Most liquid carrying containers, for example milk bottles, require a closure which tightly seals the opening of the container to prevent leakage of the contents therein. A typical container has a threaded neck with a horizontal, radially inwardly directed annular lip at the top of the neck which circumscribes the opening to the container. A typical threaded closure for use therewith is shown in Fillmore, U.S. Pat. No. 3,980,195, which includes an annular sealing ring having an outer diameter larger than the inner diameter of the container lip so that when the threaded cap is screwed onto the bottle, the sealing ring immediately engages the lip and forms a pressure seal. The ring is, essentially, forced into the smaller opening in the bottle, forming an interference type fitted seal between the annular sealing ring and the bottle lip. Typically the annular ring is compressed inwardly and the container lip is distorted downwardly under the pressure of the seal.
This commonly used seal can have several drawbacks. First, because of the use of an inteference fit, the cap may be misaligned as it is engaged to the bottle neck and considerable pressure or friction opposes the cap as it is turned onto the bottle neck. Consequently, cross threading may occur. A cross-threaded cap and bottle becomes a severe leaker due to the deformation of the plastic elements. Secondly, the interference fit causes the otherwise flat bottle lip to be distorted downwardly which prevents the machine finished edge of the bottle lip to be flush with the sealing ring of the cap. A proper seal, therefore, cannot form because the top of the bottle lip is now the engaging sealing surface, and this portion of the lip typically has a mold parting line, an irregularity in the surface of the lip, which causes a gap at the interface between the lip and sealing ring.
Another cap used to seal such containers is shown in Feldman, U.S. Pat. No. 3,901,404. Again, a primary seal is formed by the use of an oversized annular ring which forms an interference fit with the bottle lip. In order to circumvent the poor seal, Feldman utilizes a plurality of additional sealing surfaces spaced between the annular ring and the cap outer body. Though an effective seal can thus be achieved, such a cap is undesirable because of the more complex and expensive mold operations in making these caps.
It is thus apparent that the state of the art is such that the need exists for an inexpensive container and closure which will be effectively sealed against leakage of the contents and not require a complex molding operation.